1. Field of the Invention
This invention relates generally to hazardous-location-rated fixtures and specifically to illuminated exit signs.
2. Description of the Prior Art
Fixtures for use within hazardous locations are well known. In the 1920s, the National Electric Code addressed requirements for fixtures to be located in rooms or compartments in which highly flammable gases, liquids, mixtures or other substances were manufactured, used or stored. In 1931, the NEC first defined hazardous location classifications Class I for gases and vapors, Class II for dusts, and Class III/Class IV for fibers. Four years later, the NEC subdivided Class I locations into groups based on explosive pressure and flame transmission capacity. Group A includes acetylene. Group B includes hydrogen. Group C includes ethyl ether, and Group D includes gasoline, petroleum, alcohol, acetone, solvent vapors, and gases and vapors of equivalent hazard. Likewise, in 1937, the NEC defined groups for Class II with Group E including metal dusts, Group F including coal and other carbonaceous dusts, and Group G including woods, plastic, et cetera. In 1947, the NEC combined flammable fiber Classes III and IV, and it introduced the concept of divisions, where Division 1 indicates a location where ignitable concentrations (of gases vapors or liquids for Class I and of combustible dusts for Class II) can exist all of the time or some of the time under normal operating conditions and where Division 2 indicates a location where ignitable concentrations (of gases vapors or liquids for Class I and of combustible dusts for Class II) are not likely to exist under normal operating conditions. For each class, group and division, performance and construction standards for fixtures have been established to ensure safety within the hazardous area.
Today, a worldwide industry exists for setting performance standards for devices which operate within hazardous locations and for certifying those devices which meet those standards. For example, Underwriters Laboratories (UL), National Fire Protection Association (NFPA), American National Standards Institute (ANSI), National Electrical Manufacturers Association (NEMA), Canadian Standards Association (CSA), International Electrotechnical Commission (IEC), and European Committee for Electrotechnical Standardization (CENELEC) all publish standards for equipment or fixtures to be located in various hazardous locations. In 1997, in response to recent attempts at global harmonization of the various international standards, the NEC introduced the international zone classification system for Class I as an alternative to Division 1 and Division 2 classifications, which are used only in the United States and Canada. Zone 0 indicates locations where ignitable concentrations of flammable gases, vapors, or liquids are present continuously or for long periods of time under normal operating conditions. Zone 1 indicates locations where ignitable concentrations of flammable gases, vapors or liquids are likely to exist under normal operating conditions, and Zone 2 indicates locations where ignitable concentrations of flammable gases, vapors or liquids are not likely to exist under normal operating conditions. The international zone classification system has its own group ratings to indicate the potential explosive pressure and flame transmission characteristics of the hazardous area.
Although somewhat obfuscatory, the various class/division or class/zone hazardous area classifications form a framework which can be used to summarize the accepted protection methods approved for use. For example, for areas where ignitable concentrations of flammable gases, vapors or liquids can exist all of the time or some of the time under normal operating conditions (Class I Division 1, U.S. and Canada only), accepted protection methods for fixtures include explosion-proof construction, Type X or Y purging/pressurizing of the fixture, or using two-fault intrinsically safe circuitry. Where ignitable concentrations of flammable gases, vapors or liquids are not likely to exist under normal operating conditions (Class I Division 2, U.S. and Canada only), the accepted protection methods can be relaxed to include less rigorous standards, including nonincendive or non-sparking device construction, Type Z purging/pressurizing, and hermetically sealed construction.
The Class I international zone classification protection methods are similar, but some differences exist. For Class I Zone 0 (where ignitable concentrations of flammable gases, vapors, or liquids are present continuously or for long periods of time under normal operating conditions), only two-fault intrinsically safe circuitry is authorized; neither purging/pressurizing nor explosion-proof construction is deemed sufficient. Class I Zone 1 (where ignitable concentrations of flammable gases, vapors, or liquids are likely to exist under normal operating conditions) protection methods include encapsulation, flameproof construction, one-fault intrinsically safe circuitry, oil immersion, powder filling, and purging/pressurizing. Class I Zone 2 (where ignitable concentrations of flammable gases, vapors, or liquids are not likely to exist under normal operating conditions) protection methods include nonincendive or non-sparking construction, restricted breathing, hermetically sealed construction, energy limited circuitry, and simplified pressurization techniques.
Class II categories have similar protection methods. For Class II Division 1 areas (where ignitable concentrations of combustible dusts can exist all of the time or some of the time under normal operating conditions), protection methods include dust ignition-proof construction, intrinsically safe circuitry, and pressurization. Class II Division 2 (where ignitable concentrations of combustible dusts are not likely to exist under normal operating conditions) protection methods include dust-tight construction and nonincendive or non-sparking construction.
Generally, the more stringent protection methods authorized for higher (more hazardous) area classifications can be used in areas characterized by lower (less hazardous) classifications. For example, in addition to the nonincendive or non-sparking construction, restricted breathing, hermetically sealed construction, energy limited circuitry, or simplified pressurization techniques authorized for Class I Zone 2 areas mentioned above, any protection method authorized for Class I Zone 0, Class I Zone 1, or Class I Division 1 is suitable for use in Class I Zone 2 areas.
Explosion-proof exit signs which are suitable for use in Class I Division 1 Groups C and D, Class 1 Zone 0 Groups C and D, Class II Division 1 Groups E, F, and G, and Class III areas, among others, are known in the art. They typically include two incandescent lamps housed in a casing designed to withstand the pressure of explosions generated by an internal arc without propagating the explosion into the surrounding hazardous atmosphere. Inevitably, the incandescent lamps are subject to burn out, requiring maintenance and upkeep. A relamping tool is generally required to replace the light bulbs, and should the maintenance person fail to properly seal the exit sign after lamp replacement, explosion-proof integrity may be compromised.
Further, the explosion-proof exit signs known in the art do not have an integral battery back-up or other source of redundant power to keep the signs illuminated during power failures. Thus, it is generally required to wire the exit signs on a dedicated circuit which is equipped with an external redundant power source such as an emergency generator. The separate circuit(s) required for exit signs results in increased facility construction costs.
It is advantageous to have an explosion-proof exit sign which does not require periodic lamp replacement and which contains an integral battery back-up.
3. Identification of Objects of the Invention
An object of the invention is to provide an exit sign suitable for use in areas with a hazardous or potentially hazardous atmosphere which uses a long-life low-powered non-incandescent light source for maximum reliability.
Another object of the invention is to provide an exit sign suitable for use in areas with a hazardous or potentially hazardous atmosphere which eliminates the requirement for external redundant power by including an internal rechargeable battery backup.
Another object of the invention is to provide a method for aiding emergency egress from an area with a hazardous or potentially hazardous atmosphere by providing a reliable exit sign.